1. Field of the Invention
The present invention concerns an electric motor speed control in general and starting pattern control for an elevator drive motor in particular.
2. Description of the Prior Art
Many prior art elevator systems utilized a d.c. motor to drive a hoist sheave for raising and lowering the elevator cab. Typically, a.c. power was converted to d.c. power and a variable voltage control, such as a Ward-Leonard system, was utilized to control the motor at the desired speed. Where an a.c. motor has been utilized, its low starting torque has required operation at a relatively high speed with connection to the hoist sheave through a gear reduction set. Such operation produced unacceptable values of jerk and acceleration on the passengers. One attempt to overcome this problem was the use of a two speed, a.c. motor having a low speed winding for starting and a high speed winding for rated speed running. However, such a system produced an annoying jar when switching from low to high speed.
More recent developments in SCRs and thyristors have made them suitable for use in controlling motor speed. A trigger or firing angle control circuit is utilized to change the voltage applied to an induction motor to control its rotational speed from stall up to near synchronous speed. A pattern generator is utilized to generate start up, run and stopping pattern signals representing the desired motor speed which signals are compared with a signal representing the actual motor speed to generate an error signal. The error signal is utilized to adjust the firing angle of the SCRs to maintain the actual motor speed equal to the desired motor speed.
One form of speed control for an elevator system has switching logic which generates a step signal representing the desired speed which signal is compared with a function generator output signal to generate an error input signal to the function generator. The function generator integrates the error input signal to generate a ramp output signal as the speed pattern. The speed pattern signal is compared with a tachometer signal representing the elevator speed to control back-to-back SCRs which conduct power to a single speed, three phase induction motor. Braking is accomplished by applying reverse a.c. power to the motor. Such a speed control is shown in U.S. Pat. No. 3,678,355.
Another form of speed control for a three phase induction motor in an elevator system controls the acceleration at start up in accordance with the difference between the actual motor speed signal and a predetermined speed command signal. At the end of the acceleration period, the motor is run at rated speed in an open loop control configuration. Braking is accomplished by applying d.c. power to the motor. Such a speed control is shown in U.S. Pat. No. 3,973,175.